My first big question is, why is the Foundation using Class-D personnel to water the tree, especially with the risks involved by getting too close to the tree? The Foundation is an effective machine; they're not going to want to potentially waste resources when a more efficient and cost-effective automation solution, such as a sprinkler system, would work better.

Second, how does this tree get nourishment? If this is a large oak tree, why was it uprooted and brought to a containment facility? Is the floor of the containment cell dirt? I know nothing about Site-52, but I know trees at least a little, and they need more than water to sustain themselves. They need sunlight for photosynthesis, and pull many of their nutrients from the soil they dig their roots into.

Third, what is this "experimenting" you're referencing in the Containment Procedures?

Here's where we get into the bulk of the my criticism. On the whole, your clinical tone is good, but not entirely professional. One small instance is in word choice-words like "noticed" can be replaced by "observed", while phrases like "can no longer move" is an apt description, but the tone would be matched better with something akin to "becomes immobile". One is more casual, the other more observational, and observational is the point of these documents.

Further on this point, you state "When any Class-D makes contact with SCP-XXXX". Don't leave out the word "personnel" when referring to Class-D personnel. It adds to the clinical tone. Second, is this feature of the tree only true with Class-D personnel? If so, what happens to someone who is not a Class-D personnel? If this isn't true, then remove "Class-D" from this segment; something like "Any human who comes in contact with SCP-XXXX" conveys the same meaning, but without a weird (and seemingly arbitrary) distinction.

"When placed within a structure that obstructs its view of the sky, SCP-XXXX 'projects' the image of a sky and grassy field on all nearby surfaces. This projection includes an image of the sun that produces sufficient solar radiation to permit photosynthesis to occur. Tests attempting to walk past the walls into the “outside” have all failed, as the Class-D ends up running into the wall."
See, now here we get into the solar radiation thing for photosynthesis, which somehow the tree produces itself. That's good, you're thinking about ways to abnormally get around a problem. Again, though how else does it get nutrients? For a large tree like an oak tree, photosynthesis is usually only part of it—most of the nutrients are in the soil.

"D-3453: I can't move it. It's completely surrounded.

Dr. Flint: Stay calm, D-3453. Try to get your hand unstuck."

The Class-D personnel doesn't come across as frantic, there's no reason solely based on the dialogue that Dr. Flint should tell him to stay calm. However, a small change like this:
"D-3453: I can't move it! It's completely surrounded!"
adds a tone of franticness that the Dr. then has a reason to rebuke. Very small change, but on the whole, I think small changes are really what will take this from "good" to "great".

And of course, keep in mind that while I've casually skulked and read a lot of SCPs, I'm fairly new here, so anything I say should be taken as the ramblings advice of someone who tends to pick apart and analyze literary works. The moderators, they're the veterans who really know what makes a good SCP, so don't take my suggestions as gospel—wait for their feedback too.